Australian Geomechanics Vol 49 No 4 December 2014 73 ANALYSIS OF INSTALLATION FORCES FOR HELICAL PILES IN CLAY J. P. Hambleton 1 , S. A. Stanier 2 , C. Gaudin 2 , K. Todeshkejoei 1 1 ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle, Callaghan, NSW, Australia 2 ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Western Australia, Crawley, WA, Australia ABSTRACT Installation forces play a central role in the design and performance of helical piles, especially since the installation torque is often used as an indicator of the pile’s ultimate capacity. This paper presents an analytical model for predicting the installation torque for single-helix piles in clay. As an extension of a recent study by the authors, the proposed model considers not only the forces occurring on the helical plates but also the shear stresses generated along the shaft, both of which impact the installation forces. The model yields a straightforward expression that relates installation torque to the undrained shear strength of the soil, embedment depth, helix diameter and pitch, shaft diameter, crowd (axial) force, and adhesion coefficient along the shaft. The influence of these factors on the installation torque, as well as the “capacity-to-torque ratio” used to infer capacity from the installation, is assessed through a sensitivity analysis. Some level of validation is provided through a comparison with empirical capacity-to-torque ratios, and the sensitivity analysis reveals factors that are neglected in empirical models but nevertheless have a significant influence. 1 INTRODUCTION Helical piles are deep foundations consisting of one or more helical plates mounted to a central shaft. Their main feature is the method of installation, which involves twisting the pile into the ground under an applied torque and crowd (axial) force. On account of the relative ease of the installation process compared to traditional deep foundations (e.g., driven and drilled piles) and growing acceptance within the geotechnical engineering community, the popularity of helical piles has risen markedly over the past few decades. Figure 1 depicts several applications where helical piles are routinely used. Apart from these well-established applications, helical anchors have been identified as a promising foundation solution for near-shore wave energy converters. Common alternatives to the term “helical pile” include “screw piles” and “helical anchors,” the latter being reserved for applications involving uplift (e.g., Figures 1b, 1c, and 1d). As with most types of foundation, the vast majority of the theory developed for helical piles focuses on the uplift or bearing capacity of the configuration at its final embedment depth, without regard for the process of installation. The rigorous analyses conducted by Merifield (2011) and Wang et al. (2013), discussed further in the companion paper in this issue (Gaudin et al., 2014) are examples where such “wished-in-place” conditions are assumed. The monograph by Perko (2009) provides a complete overview of the prevailing methods for predicting uplift and bearing capacity of helical piles alongside a summary of their historical development, design issues, example applications, practical aspects of installation, field testing and loading conditions. Despite the recognised impacts of the installation process on the performance of helical piles (cf. Perko 2009; Tsuha et al., 2012), and in particular the widely adopted use of installation torque as an indicator of ultimate capacity, relatively few attempts have been made to model the installation process theoretically. This can be attributed to the degree of difficulty, since the installation process involves large, predominantly plastic deformation, contact interaction at the soil-helix and soil-shaft interfaces, and material separation at the leading edge of the helical plates as they cut the soil. Ghaly et al. (1991) and Tsuha and Aoki (2010) proposed analytical models for predicting the installation torque for helical piles in sand, and Perko (2000) developed an approximate model that directly relates the bearing or uplift capacity to the installation torque via balance of energy. Whereas the former models pertain specifically to sand, an acknowledged peculiarity of Perko’s model is the absence of soil strength parameters. Perko’s model is nevertheless consistent with previously established empirical models that relate the bearing or uplift capacity to the installation torque without consideration for the material type. In particular, the widely accepted method referred to here as “torque- capacity correlation” relies on the following empirical formula (Hoyt and Clemence, 1989) max  F KT (1)